1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device having a self-aligned contact pad.
2. Description Related Art
Generally, if the degree of the integration of a semiconductor device is increased by reducing design dimensions in the semiconductor device, tolerance for the alignment of a contact hole with respect to lower semiconductor layers and lower wiring can become increasingly difficult.
Heretofore, a method of forming a contact pad using a selective anisotropic epitaxial silicon growth technique to solve such a problem was known. The above forming method using the selective anisotropic epitaxial silicon growth technique is described, for example, by Hada et al, "A Self-Aligned Contact Technology Using Anisotropic Selective Epitaxial Silicon For Giga-Bit DRAMs," IEDM Technical Digest, p.665, 1995.
FIG. 1 shows an arrangement wherein corners portions 2 of a diffusion layer 1 are exposed between word lines 3. The shape of the diffusion layer 1 is rectangular. (Diffusion layer 1 is an element forming region having portions into which dopants are diffused.) Crystal orientations of the long sides and the short sides of the diffusion layer 1 are arranged &lt;110&gt;. In this case, silicon is selectively grown on a diffusion layer 18. For example, the silicon is selectively subjected to anisotropic epitaxial growth using Si.sub.2 H.sub.6 gas and PH.sub.3 gas, setting the flow rate of Si.sub.2 H.sub.6 gas to 1 cc per minute and setting the temperature of a silicon substrate to 700.degree. C.
But if the corner portion 2 of the diffusion layer 1 is rounded, the crystal orientation of the corner portion 2 does not become &lt;110&gt;. As a result, ideal anisotropic growth may not occur and isotropic growth may occur.
FIG. 2 is a sectional view taken along the line II--II, FIG. 2 shows that if the silicon epitaxial growth is isotropic growth, there is a problem that epitaxial silicon 8 laterally grown is grown on element separating regions 5, and brought into contact with each other to cause a short circuit.
Moreover, as shown in FIG. 3, a crystal plane (facet) having crystal orientation other than [100] may be generated. If such facets are generated, as shown in FIG. 3, there is a problem that both upper parts of first facets 19 and second facets 20 are connected during growth to thereby block further vertical epitaxial growth.